Kinesins are motor proteins that hydrolyze adenosine triphosphate as they travel along microtubules and generate mechanical force. These proteins are characterized by containing a motor domain having about 350 amino acid residues. The crystal structures of several kinesin motor domains have been resolved.
Currently, about one hundred kinesin-related proteins (KRP) have been identified. Kinesins are involved in a variety of cell biological processes including transport of organelles and vesicles, and maintenance of the endoplasmic reticulum. Several KRP's interact with the microtubules of the mitotic spindle or with the chromosomes directly and appear to play a pivotal role during the mitotic stages of the cell cycle. These mitotic KRP's are of particular interest for the development of cancer therapeutics.
Kinesin spindle protein (KSP) (also known as Eg5, HsEg5, KNSL1, or KIF11) is one of several kinesin-like motor proteins that are localized to the mitotic spindle and known to be required for formation and/or function of the bipolar mitotic spindle.
In 1995, the depletion of KSP using an antibody directed against the C-terminus of KSP was shown to arrest HeLa cells in mitosis with monoastral microtubule arrays (Blangy et al., Cell 83:1159-1169, 1995). Mutations in bimC and cut7 genes, which are considered to be homologues of KSP, cause failure in centrosome separation in Aspergillus nidulans (Enos, A. P., and N. R. Morris, Cell 60:1019-1027, 1990) and Schizosaccharomyces pombe (Hagan, I., and M. Yanagida, Nature 347:563-566, 1990). Treatment of cells with either ATRA (all trans-retinoic acid), which reduces KSP expression on the protein level, or depletion of KSP using antisense oligonucleotides revealed a significant growth inhibition in DAN-G pancreatic carcinoma cells indicating that KSP might be involved in the antiproliferative action of all trans-retinoic acid (Kaiser, A., et al., J. Biol. Chem. 274, 18925-18931, 1999). Interestingly, the Xenopus laevis Aurora-related protein kinase pEg2 was shown to associate and phosphorylate XlEg5 (Giet, R., et al., J. Biol. Chem. 274:15005-15013, 1999). Potential substrates of Aurora-related kinases are of particular interest for cancer drug development. For example, Aurora 1 and 2 kinases are over expressed on the protein and RNA level and the genes are amplified in colon cancer patients.
The first cell permeable small molecule inhibitor for KSP, “monoastral,” was shown to arrest cells with monopolar spindles without affecting microtubule polymerization as do conventional chemotherapeutics such as taxanes and vinca alkaloids (Mayer, T. U., et al., Science 286:971-974, 1999). Monastrol was identified as an inhibitor in phenotype-based screens and it was suggested that this compound may serve as a lead for the development of anticancer drugs. The inhibition was determined not to be competitive with respect to adenosine triphosphate interaction with KSP, and was found to be rapidly reversible (DeBonis, S., et al., Biochemistry 42:338-349, 2003; Kapoor, T. M., et al., J. Cell Biol. 150:975-988, 2000).
In light of the importance of improved chemotherapeutics, there is a need for KSP inhibitors that are effective in vivo inhibitors of KSP and KSP-related proteins. Some inhibitors of KSP have been reported previously. For example, WO 06/002236 and PCT/US2006/031129 disclose certain classes of compounds indicated to be inhibitors of KSP. Ispinesib (SB-715992) is a clinical candidate from Cytokinetics that is indicated to act as a KSP inhibitor. The present invention provides new KSP inhibitors with improved activities and new methods of using these KSP inhibitors. In addition, it provides novel KSP inhibitors that are effective against cancer cells that are resistant to other therapeutic agents such as paclitaxel due to their expression of P-glycoprotein that acts as an efflux pump.